Solid-polymer-electrolyte membrane for fuel cell and process for producing the same

ABSTRACT

A solid-polymer-electrolyte membrane for a polymer-electrolyte fuel cell is formed of a synthetic resin. The synthetic resin includes a main chain, and a hydrocarbon-based side chain. The main chain is formed as a film, and formed of a copolymer made from a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer. The hydrocarbon-based side chain involves a sulfonic group. The solid-polymer-electrolyte membrane exhibits a high strength and flexibility, but a low electric resistance, and can be produced at a reduced manufacturing cost. Thus, the solid-polymer-electrolyte membrane can be effectively applied to construct polymer-electrolyte fuel cells.

This application is a division of Ser. No. 08/688,769 filed Jul. 31,1996, now U.S. Pat. No. 5,817,718.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-polymer-electrolyte membranefor a polymer-electrolyte fuel cell, and a process for producing thesame.

2. Description of Related Art

A polymer-electrolyte fuel cell employs hydrogen and oxygen as a fueland an oxidizing agent, respectively, and is considered a promisingsmall-and-lightweight power source to be applied to automobiles, etc.Such a fuel cell comprises a solid-polymer-electrolyte membrane havingopposite surfaces, a positive electrode disposed in contact with one ofthe opposite surfaces of the membrane, and a negative electrode disposedin contact with another one of the opposite surfaces of the membrane.Hydrogen is oxidized electrochemically at the negative electrode toproduce protons and electrons. The protons are transferred through thesolid-polymer-electrolyte membrane to the positive electrode, to whichoxygen is supplied. Whilst, the electrons produced at the negativeelectrode flow by way of a load, which is connected to the fuel cell, tothe positive electrode. At the positive electrode, the electrons reactwith protons and oxygen to produce water.

It has been known that the performance of a fuel cell depends greatly onthe performance of the gas diffusion electrodes used as the positive andnegative electrodes, and on the performance of solid-polymer-electrolytemembrane. The performance required for solid-polymer-electrolytemembrane is to permit as many protons as possible to flow. In order toestablish the required performance, it has been known that it isimportant to introduce as many sulfonic groups as possible which arecapable of imparting and ion-exchange capability tosolid-polymer-electrolyte membrane.

A fluorocarbon-based resin, for example, Nafion (Trade Mark) and itsderivatives, has been known as a few of the representatives of thesolid-polymer-electrolyte membranes for the fuel-cell application.Nafion is based on a copolymer made from tetrafluoroethylene andperfluorovinylether, and is provided with sulfonic groups working asion-exchanging groups.

However, solid-polymer-electrolyte membranes formed of Nafion, and thelike, liquify when the sulfonic groups, working as ion-exchanginggroups, are introduced into the membrane in an increasing quantity inorder to decrease the electric resistance of the membranes. Thus, thesulfonic groups should be introduced into the membranes in a limitedquantity. Moreover, as the quantity of the introduced sulfonic groupsincreases, the strength of the resulting membranes degrades. When themembranes have a low electric resistance, they suffer from a problem inthat they break during operation of the fuel cells. Due to thesereasons, membranes formed of Nafion, and the like, exhibit anion-exchanging capacity of 1.1 milli-equivalent/g at the highest. Thus,it has been desired that the membranes be further improved in terms ofion conductivity.

Ion-exchange membranes formed of Nafion, and the like, have been putinto practical applications in the field of brine electrolysis industry,and have been known to have a good chemical stability. However, Nafion,and the like, are very expensive, because they are fluorocarbon-basedresins. Considering the application of polymer-electrolyte fuel cell toautomobiles, it is required that the current cost ofsolid-polymer-electrolyte membrane formed of Nafion be decreased byabout one-to-a couple of dozens to one-to-a couple of hundreds. If not,the fuel cell can hardly be believed to be put into practicalapplications.

There are other approaches for preparing solid-polymer-electrolytemembranes: namely; side chains, into which sulfonic groups can beintroduced, are brought into base films by radiation-graftpolymerization. For instance, styrene, or the like, is graft-polymerizedinto a Teflon (Trade Mark) membrane or a Teflon-based copolymer film,and thereafter sulfonic groups are introduced into the resultinggraft-polymerized polystyrene chain. The solid-polymer-electrolytemembranes prepared by this process cannot contribute to attainingsufficient fuel cell performance because of the problems hereinafterdescribed.

As described in Electrochimica, Acta 40,345 (1995), a fuel cell wasprepared by using a solid-polymer-electrolyte membrane. The membrane wasprepared by graft-polymerizing styrene ontotetrafluoroethylene-hexafluoropropylene (i.e., FEP) copolymer filmexposed to gamma radiation. Sulfonic groups were then introduced intothe thus graft-polymerized copolymer film to prepare the membrane, andthe resulting membrane was incorporated into a fuel cell. The literaturereports that, immediately after operating the fuel cell, the membranewas decomposed, and thereby the sulfonic groups were eliminated. As aresult, the internal resistance of the fuel cell was increased, and theperformance thereof was deteriorated sharply even after operating itsfuel cell for couple of dozens of hours. The literature also refers tothe fact that the fuel cell lacks sufficient output performance becausethe membrane was inferior to Nafion in terms of ion conductivity. Theliterature further reveals that the inadequate chemical stability of thegraft-polymerized polystyrene side chains resulted in the decompositionof the membrane under the operating conditions of the fuel cell.

In addition, other processes have been known for preparing polymerion-exchange membranes, and are premised on the recognition thatsulfonated polystyrene side chains have insufficient chemical stability.According to these processes, α, β, β-trifluorostyrene, one offluorinated styrenes, is graft-polymerized into a tetrafluoroethylenepolymer membrane or a tetrafluoroethylene-based copolymer membrane, andthe graft-polymerized membrane is sulfonated to prepare a polymerion-exchange membrane. For example, see U.S. Pat. No. 4,012,303 and U.S.Pat. No. 4,605,685. These U.S. patents do not specifically describe theoperating characteristics of the resulting polymer ion-exchangemembranes which are applied to a polymer-electrolyte fuel cell. Theinventors of the present invention, however, estimate that the membranessuffer from the following problems.

Firstly, fluorinated styrene or α, β, β-trifluorostyrene pushes up themanufacturing cost, because it is difficult to synthesize fluorinatedstyrene or α, β, β-trifluorostyrene. Thus, similar to the problemassociated with Nafion, and the like, the cost problem hinders thepolymer ion-exchange membranes from the application tosolid-polymer-electrolyte fuel cell.

Secondly, in the radiation-graft polymerization of α, β,β-trifluorostyrene, the reactivity is so low that α, β,β-trifluorostyrene can be introduced limitedly into thetetrafluoroethylene polymer membrane or tetrafluoroethylene-basedcopolymer membrane in an amount of 50% by weight or less. As a result,the sulfonic groups cannot be introduced into the graft-polymerizedmembrane in a large amount. Hence, in the application tosolid-polymer-electrolyte fuel cell, the membranes lack sufficient ionconductivity, similar to the membranes formed of Nafion, and the like.Thus, the membranes cannot solve the problems associated with themembranes formed of Nafion, and the like.

Thirdly, the membranes exhibit such a degree of flexibility that theyare likely to break during preparation or in the operation of a fuelcell.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the aforementionedcircumstances. It is therefore an object of the present invention toprovide a solid-polymer-electrolyte membrane which can be produced at areduced cost, has a high ion conductivity, a high durability, and asatisfactory strength.

Extensive research has now been conducted on radiation-graftpolymerization processes for preparing a solid-polymer-electrolytemembrane which has suitable performance for use in polymer-electrolytefuel cells. We have discovered the following facts:

1) When non-hydrogen containing; completely fluorinated polymermembranes such as tetrafluoroethylene-hexafluoropropylene (i.e., FEP)copolymer are irradiated with gamma-rays or the like, thereby producingradicals, and then a polymerizable vinyl compound such as styrene, isgraft-polymerized onto the membrane, and thereafter sulfonic acid groupsare introduced into the graft-polymerized membrane by a sulfonatingreaction to prepare a solid polymer-electrolyte membrane, the thusprepared membranes are very brittle.

First of all, when the completely fluorinated polymer membrane wasirradiated by a gamma ray, or the like, the irradiated membrane showed aconsiderable strength deterioration. When the irradiated membrane wasfurther subjected to the graft-polymerization reaction and thesulfonating reaction, the strength of the resulting membrane was furtherdegraded. Hence, when a membrane having an ion conductivity was preparedby using a completely fluorinated polymer membrane, the resultingmembrane was so brittle that it broke in the preparation or operation offuel cell. In fact, it was impossible to prepare a fuel cell.

In order to solve the problem of loss of strength, attempts were made toirradiate polymer membranes at reduced doses. It was, therefore,possible to prepare a fuel cell using the resulting membrane, and tooperate the fuel cell. However, immediately after operating the fuelcell, the fuel cell exhibited an appreciable internal resistanceincrement. Thus, the resulting membrane had apparently decomposed.

2) When a polytetrafluoroethylene (i.e., PTFE) polymer membrane or atetrafluoroethylene-perfluoroalkylvinylether (i.e., PFA) copolymermembrane was employed as a base film for the radiation-graftpolymerization, phenomena similar to the above 1) could be observed moreapparently. In fact, it was impossible to prepare a fuel cell by usingthe PTFE polymer or PFA copolymer membrane.

3) A polymer membrane involving hydrocarbon, such as anethylene-tetrafluoroethylene (i.e., ETFE) copolymer membrane, wassubjected to the same radiation-graft polymerization in order tograft-polymerize styrene, and the graft-polymerized membrane wassulfonated to prepare a solid-polymer-electrolyte membrane. Contrary tothe above 1) and 2), the resulting membrane maintained the strengthwhich was sufficient to prepare and operate a fuel cell. Moreover, afteroperating fuel cells employing the membrane for hundreds of hours, themembrane did not deteriorate.

The aforementioned discovery recited in the above 3) was one of thebases for completing the present invention. It is important as well,because the decomposition of styrene-graft-polymerized membranesprepared by the radiation-graft polymerization was not caused by themechanism which has been contemplated in the related arts. Specifically,the discovery means that the decomposition of styrene-graft-polymerizedmembranes does not result from the decomposition of graft-polymerizedstyrene side chains, but depends on the properties of base films.

As a result, it is unnecessary to use fluorinated styrene or α, β,β-trifluorostyrene to compensate for the chemical stability ofgraft-polymerized styrene side chains. Accordingly, it is possible tosolve the problems (e.g., high cost, improper ion conductivity, etc.)associated with α, β, β-trifluorostyrene.

In addition, the discovery also indicates that a polymer membraneinvolving a hydrocarbon, such as an ethylene-tetrafluoroethylene (i.e.,ETFE) copolymer membrane, does not deteriorate considerably uponexposure to radiation.

We, the inventors of the present invention, continued the research anddevelopment on the differences between the completely fluorinatedpolymer membrane and the polymer membrane involving unfluorinatedhydrocarbon segments which were subjected to the radiation-graftpolymerization. We further discovered the facts described below.

4) In the completely fluorinated polymer film, the cleavages occurreddominantly in the main chains by the energy of the irradiated ray, andthe graft-polymerized styrene side chains were bonded tolow-molecular-weight fragments resulting from the cleaved main chains.The low-molecular-weight fragments with the graft-polymerized styreneside chains bonded thereto dissolved and disappeared into water duringthe operation of fuel cell.

5) On the other hand, in the polymer film partially involving thehydrocarbon segments in the fluorinated carbon skeletons, thehydrocarbon segments, present in the main chains of skeletons, werecross-linked intermolecularly upon irradiation. As a result, even whenthe main chains were broken at the fluorinated carbon skeletons, thegraft-polymerized styrene side chains were still bonded to thecross-linked macromolecules. Hence, in this case, the macromoleculeswith the graft-polymerized styrene side chains bonded did not dissolveand disappear into water in the operation of the fuel cell.

Based on the results of the above-described researches and developments,the inventors completed the following present solid-polymer-electrolytemembrane for a polymer-electrolyte fuel cell.

The present solid-polymer-electrolyte membrane for a polymer-electrolytefuel cell is formed of a synthetic resin which comprises:

a main chain formed as a film, and formed of a copolymer made from afluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer;and

a hydrocarbon-based side chain involving a sulfonic group.

In accordance with the present invention, a process is provided forproducing the present solid-polymer-electrolyte membrane for apolymer-electrolyte fuel cell. The process comprises the steps of:

irradiating a film-shaped copolymer made from a fluorocarbon-based vinylmonomer and a hydrocarbon-based vinyl monomer, and thereafter contactinga polymerizable alkenyl benzene with the irradiated copolymer, therebyforming a graft side chains resulting from the polymerizable alkenylbenzene; and

introducing a sulfonic group into the resulting graft side chain.

Moreover, a modified version of the present process is provided forproducing the present solid-polymer-electrolyte membrane for apolymer-electrolyte fuel cell. The modified version comprises:

irradiating a film-shaped copolymer made from a fluorocarbon-based vinylmonomer and a hydrocarbon-based vinyl monomer, and thereafter contactinga polymerizable alkenyl benzene having a sulfonic group with theirradiated copolymer, thereby forming a graft side chain resulting fromthe polymerizable alkenyl benzene having a sulfonic group.

The present solid-polymer-electrolyte membrane comprises a main chain,and a hydrocarbon-based side chain. The main chain is formed of acopolymer made from a fluorocarbon-based vinyl monomer and ahydrocarbon-based vinyl monomer. The hydrocarbon-based side chaininvolves a sulfonic group.

The main chain constitutes a major portion of the presentsolid-polymer-electrolyte membrane. Into the main chain, agraft-polymerized side chain is formed. The main chain includes afluorocarbon segment, and a hydrocarbon segment. When the fluorocarbonsegment is irradiated, it is believed to produce radicals which mainlyconstitute the starting points of graft polymerization. When thehydrocarbon segment is irradiated, it also produces radicals whichconstitute the starting points of graft polymerization, but part of theproduced radicals are recombined with each other to form cross-linkedconstructions. As a result, even if the fluorocarbon segment is brokenby the irradiation, the main chain per se is cross-linked and bondedcontinuously by the cross-linked hydrocarbon segment. Hence, it isbelieved that the strength of the membrane can be maintainedsatisfactorily high, and simultaneously a high grafting rate can beattained.

The copolymer made from a fluorocarbon-based vinyl monomer and ahydrocarbon-based vinyl monomer (hereinafter simply referred to as the"present main-chain resin") can be one expressed by the followingchemical formula (1): ##STR1## In the present main-chain resin, R¹ canbe a fluorine atom or a fluoroalkyl group whole number of carbon atomsis from 1 to 3, R² can be a hydrogen atom or an alkyl group whose numberof carbon atoms is from 1 to 3, m can be an integer of 1 or more, and ncan be an integer of 1 or more. It is further preferred that the numberof carbon atoms in R¹ and R² can be 3 or less. It is furthermorepreferred that R¹ can be a fluorine atom, and R² can be a hydrogen atom.Note that, in the present main-chain resin, the number of structuralunits, m and n, can be large enough to form a solid polymer.

The present main-chain resin is available in a variety of forms. In thepresent invention, it is formed as a membrane. Note that the thicknessof the membrane is not limited in particular, and that the thickness canbe adjusted so as to satisfy the objective, for example, of establishinga sufficient strength and a low electric resistance.

The side chain, which is introduced into or branched from the presentmain-chain resin, is a hydrocarbon-based side chain containing asulfonic group (hereinafter simply referred to as the "presentside-chain resin"). The present side-chain resin can be one expressed bythe following chemical formula (2): ##STR2## The present side-chainresin is a component for introducing an ion-exchanging function into thepresent solid-polymer-electrolyte membrane. In the present side-chainresin, R³, R⁴, and R⁵ can be a hydrogen atom or an alkyl group whosenumber of carbon atoms is from 1 to 3, respectively. When the number ofcarbon atoms in R³, R⁴, and R⁵ of the present side-chain resin is toomany, the resulting side chain cannot be formed satisfactorily, or thepolymerization cannot be fully developed. Therefore, it is preferredthat the number of carbon atoms in R³, R⁴, and R⁵ can be 3 or less. Itis further preferred that R³ and R⁴ can be a hydrogen atom,respectively, and R⁵ can be a hydrogen atom or an alkyl group.

In the present side-chain resin, t can be 0 or an integer of 1 or more,especially t can preferably be 0. It is most preferred that R³, R⁴, andR⁵ can be a hydrogen atom, respectively, and t can be 0: namely; this isthe case where styrene having a sulfonic group results in the presentgraft-polymerized side-chain resin.

Note that, however, s is determined as desired so that the resultingpresent solid-polymer-electrolyte membrane can satisfactorily attain agrafting rate, an ion-exchange capacity, and a water content, andaccordingly has a value of at least one.

The hydrocarbon-based side chain containing a sulfonic group can beprepared by graft-polymerizing a polymerizable alkenyl benzene, or apolymerizable alkenyl benzene containing a sulfonic group, onto thecopolymer, or the present main-chain resin which is prepared from afluorocarbon vinyl monomer and a hydrocarbon-vinyl monomer. Thepolymerizable alkenyl benzene can be one expressed by the followingchemical formula (3): ##STR3## The polymerizable alkenyl benzene havinga sulfonic group can be one expressed by the following chemical formula(4): ##STR4## In the chemical formulas (3) and (4), R³, R⁴, and R⁵ canbe a hydrogen atom or an alkyl group whose number of carbon atoms isfrom 1 to 3, respectively, and t can be 0 or an integer of 1 or more.

A grafting rate of the resulting present solid-polymer-electrolytemembrane can be calculated by the following equation: ##EQU1##

In order to give proper ion-exchange capacity and water content to thepresent solid-polymer electrolyte membrane, a grafting rate canpreferably be controlled to 10% or more, further preferably 20% or more.Moreover, in order to give adequate tensile strength, elongation, gasrepellency, bondability to electrodes, and oxidation resistance to thepresent solid-polymer electrolyte membrane, a grafting rate canpreferably be controlled to 150% or less, further preferably 130% orless, furthermore preferably 100% or less.

An ion-exchange capacity of the resulting presentsolid-polymer-electrolyte membrane can be calculated by the followingequation: ##EQU2##

In order to give proper water content and low electric resistance to thepresent solid-polymer electrolyte membrane, an ion-exchange capacity canpreferably be controlled to 1.0 milli-equivalent/g or more, furtherpreferably 1.2 milli-equivalent/g or more. Moreover, in order to giveadequate tensile strength, elongation, bondability to electrodes, andoxidation resistance to the present solid-polymer-electrolyte membrane,an ion-exchange capacity can preferably be controlled to 3.5milli-equivalent/g or less, further preferably 3.0 milli-equivalent/g orless.

A water content of the resulting present solid-polymer-electrolytemembrane can be calculated by the following equation: ##EQU3##

In order to let a fuel cell employing the presentsolid-polymer-electrolyte membrane generate a sufficient power, a watercontent can preferably be controlled to 30% or more, further preferably50% or more. Moreover, in order to inhibit the presentsolid-electrolyte-membrane from remarkably enlarging dimensionally, andto keep its strength properly, a water content can preferably becontrolled to 200% or less, further preferably 150% or less.

The present solid-polymer-electrolyte membrane can be produced by eitherone of the following two processes. According to one process, acopolymer made from the fluorocarbon vinyl monomer and the hydrocarbonvinyl monomer is irradiated, and the irradiated copolymer is contactedwith the polymerizable alkenyl benzene. Thus, the graft-polymer sidechain resulting from the polymerizable alkenyl benzene is formed.Thereafter, a sulfonic group is introduced into the graft-polymer sidechain. According to another process, instead of a polymerizable alkenylbenzene, the above-described polymerizable alkenyl benzene having asulfonic group is contacted with a copolymer made from thefluorocarbon-based vinyl monomer and the hydrocarbon-based vinylmonomer. This latter process obviates the subsequent sulfonating step,because the graft-polymer has a sulfonic group already.

When a copolymer made from the fluorocarbon-based vinyl monomer and thehydrocarbon-based vinyl monomer is irradiated by a ray, the copolymerproduces radicals therein. The radicals react with the polymerizablealkenyl benzene, or the polymerizable alkenyl benzene having a sulfonicgroup. As a result, the graft-polymer side chain is formed.

When a dose of the irradiation is too less, the graft-polymer side chaindoes not grow satisfactorily. Therefore, a ray can be irradiated to acopolymer at a dose of 1 kGy or more, preferably 5 kGy or more, furtherpreferably 10 kGy or more. When a dose of the irradiation is too much, acopolymer, made from the fluorocarbon-based vinyl monomer and thehydrocarbon-based vinyl monomer, is broken. Consequently, the resultingmembranes are so brittle that they are not applicable to fuel cells.Hence, a copolymer can be irradiated at a dose of 100 kGy or less,preferably 80 kGy or less, further preferably 50 kGy or less.

In the present invention, any ray can be employed for the irradiationfreely. For instance, a gamma ray or an accelerated electron beam can beemployed therefor.

A copolymer made from the fluorocarbon-based vinyl monomer and thehydrocarbon-based vinyl monomer can be contacted with the polymerizablealkenyl benzene, or the polymerizable alkenyl benzene having a sulfonicgroup in vacuum, in inert-gas atmosphere, or in air. Note that, however,the contacting operation can preferably be carried out in vacuum, or ininert-gas atmosphere, in order to inhibit radicals from beingextinguished by oxygen and to let the graft-polymer side chain growsufficiently. In the contacting operation, the reaction temperature canpreferably be controlled to a boiling point or less of the polymerizablealkenyl benzene in order that the resulting present membrane can hold asufficient strength.

A sulfonic group can be introduced into the graft-polymerized membrane,resulting from a copolymer made from the fluorocarbon-based vinylmonomer and the hydrocarbon-based vinyl monomer, and the polymerizablealkenyl benzene, by using a variety of sulfonating agents in vacuum, ininert-gas atmosphere, or in air. Above all, in order to efficiently andfully introduce a sulfonic group thereinto, and to give a satisfactorystrength to the present solid-polymer-electrolyte membrane, a sulfonicgroup can preferably be introduced thereinto by using chlorosulfonicacid or fluorosulfonic acid in vacuo or in inert-gas atmosphere. Amongthe sulfonating agents, chlorosulfonic acid is most preferred. In thesulfonic-group introducing operation, the reaction temperature canpreferably be controlled to 50° C. or less in order that the resultingpresent membrane can hold a sufficient strength.

As having been described so far, contrary to conventionalsolid-polymer-electrolyte membranes, the thus prepared presentsolid-polymer-electrolyte membrane exhibits a high tensile strength andflexibility. Further, when it is combined with gas diffusion electrodesto construct fuel cells, an overall electric resistance of the resultingfuel cells is low. Furthermore, its manufacturing cost is much lessexpensive than that of commercially available Nafion. Thus, it can beeffectively applied to construct polymer-electrolyte fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of itsadvantages will be readily obtained a the same becomes better understoodby reference to the following detailed description when considered inconnection with the accompanying drawings and detailed specification,all of which forms a part of the disclosure:

FIG. 1 illustrates a construction of a fuel cell in cross-section, fuelcell which was constructed by employing a preferred embodiment of thepresent solid-polymer-electrolyte membrane;

FIG. 2 is a schematic diagram for illustrating an apparatus forexamining the performance of fuel cells;

FIG. 3 is a graph for illustrating the cell performance of fuel cells bythe relationships between output voltages and electric currentdensities, fuel cells which were constructed by employing preferredembodiments of the present solid-polymer-electrolyte membrane, andconventional solid-polymer-electrolyte membranes; and

FIG. 4 is a graph for illustrating the durability of fuel cells by therelationships between output voltages and time elapsed, fuel cells whichwere constructed by employing a preferred embodiment of the presentsolid-polymer-electrolyte membrane, and a conventionalsolid-polymer-electrolyte membrane.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having generally described the present invention, a furtherunderstanding can be obtained by reference to the specific preferredembodiments which are provided herein for the purpose of illustrationonly and not intended to limit the scope of the appended claims.

First Preferred Embodiment

An ethylene-tetrafluoroethylene (ETFE) copolymer film was washed withacetone, and was irradiated by a gamma ray at a dose of 2 kGy. Note thatthe ETFE copolymer film had a thickness of 50 μm, and that theirradiation source was Co-60.

The irradiated ETFE copolymer film was charged in a glass reactor tube,and thereafter styrene was put therein in an amount of 25 ml. Afterfully substituting nitrogen for the air in the reactor tube, the reactortube was immersed into a constant-temperature bath whose temperature hadbeen adjusted to 60° C. for 19 hours, thereby carrying out the graftpolymerization.

After the radiation-graft polymerization reaction, the ETFE copolymerfilm was washed with benzene three times, and was put in a drier to dry.

The grafted film was then immersed into a mixture solution at roomtemperature for 30 minutes. The mixture solution included 30 parts byweight of chlorosulfonic acid, and 70 parts by weight of1,1,2,2-tetrachloroethane. Thereafter, the film was taken out of themixture solution, was washed with 1,1,2,2-tetrachloroethane, and wasfurther washed with ion-exchanged water. Moreover, the film was immersedinto a 2 N KOH aqueous solution at 100° C. for 30 minutes, andthereafter was further immersed into a 1 N H₂ SO₄ aqueous solution at100° C. for 30 minutes. Finally, the film was fully washed withion-exchanged water. A First Preferred Embodiment of the presentsolid-polymer-electrolyte membrane was thus prepared.

The grafting rate, strength, specific resistance and ion-exchangecapacity properties of the resulting membrane were determined. Theresults of the examination are set forth in Table 1 below.

Note that the grafting rate was determined by weighing the ETFEcopolymer film before and after the graft polymerization in theabove-described manner. The strength of the membrane was evaluated basedon the criteria described below.

Criteria on Strength of Membrane

A: Membranes endured against a pressure of 80 kgf/cm² required forpressing and bonding gas diffusion electrodes at 165° C.;

B: Membranes broke sometimes under the pressing condition;

C: Membranes always broke under the pressing condition; and

D: Membranes were deteriorated considerably after the graftpolymerization, and broken in the subsequent operation.

Measurement of Specific Resistance

The resulting membranes were immersed into a 1 N NaCl aqueous solution,and were examined for the specific resistance by using a multi-meter(LCR meter 4261 made by YOKOGAWA HEWLETT PACKARD, Co., Ltd.).

Measurement of Ion-exchange Capacity

The resulting membranes were immersed into a 1 N HCl aqueous solution at50° C. for 10 minutes, and were washed with ion-exchanged water.Thereafter, the membranes were immersed into a 2 N NaCl aqueous solutionat 50° C. for 5 minutes, thereby expelling hydrogen ions out of themembranes into the NaCl aqueous solution. Eventually, the thus expelledhydrogen ions were titrated by neutralization with an NaOH aqueoussolution to determine the ion-exchange capacity of membranes.

                  TABLE 1                                                         ______________________________________                                                                                   Ion-ex-                                    change                                                                        Capa-                                                                     Gamma-    city                                                                Ray Grafting  Specific (milli-                                              Identi- Polymer Dose Rate  Resistance equival-                                fication Film (kGY) (%) Strength (Ω · cm) ent/g)             ______________________________________                                        1st Pref.                                                                            ETFE    2        24    A     212    1.3                                  Embodi-                                                                       ment                                                                          2nd Pref. ETFE 4 34 A 129  1.6                                                Embodi-                                                                       ment                                                                          3rd Pref. ETFE 6 40 A 69 1.7                                                  Embodi-                                                                       ment                                                                          4th Pref. ETFE 8 50 A 40 1.9                                                  Embodi-                                                                       ment                                                                          5th Pref. ETFE 10 54 A 32 2.0                                                 Embodi-                                                                       ment                                                                          6th Pref. ETFE 20 61 A   6.8 2.7                                              Embodi-                                                                       ment                                                                          7th Pref. ETFE 20 47 A 18 2.3                                                 Embodi-                                                                       ment                                                                          8th Pref. ETFE 25 88 A 15 2.7                                                 Embodi-                                                                       ment                                                                          9th Pref. ETFE 100 115 B 14 2.7                                               Embodi-                                                                       ment                                                                          Comp. Ex. PTFE 25 15 C 82 1.0                                                 No. 1                                                                         Comp. Ex. PTFE 100 * D * *                                                    No. 2                                                                         Comp. Ex. FEP 25 51 B 52 2.0                                                  No. 3                                                                         Comp. Ex. FEP 100 53 D * *                                                    No. 4                                                                         Comp. Ex. PFA 25 53 C 78 2.0                                                  No. 5                                                                         Comp. Ex. PFA 100 49 D * *                                                    No. 6                                                                       ______________________________________                                         Note)                                                                         "ETFE" means an ethylenetetrafloroethylene copolymer film.                    "PTFE" means a tetrafluoroethylene polymer film.                              "FEP" means a tetrafluoroethylenehexafluoropropylene copolymer film.          "PFA" means a tetrafloroethyleneperfloroalkylvinylether copolymer film.       The physical properties specified with * could not be evaluated because       the films broke or lost during the examinations.                         

Second through Ninth Preferred Embodiment and Comparative Example Nos. 1through 6

Except that a gamma-ray dose was varied as set forth in Table 1 above,Second through Ninth Preferred Embodiments of the presentsolid-polymer-electrolyte membrane were prepared in the same manner asthe First Preferred Embodiment.

Except that a copolymer film and a gamma-ray dose were varied as recitedin Table 1 above, Comparative Example Nos. 1 through 6 were prepared inthe same manner as the First Preferred Embodiment.

The thus prepared solid-polymer-electrolyte membranes were examined fortheir physical properties in the same manner as the First PreferredEmbodiment, and the results of the examination are summarized in Table 1above.

It is evident from the results set forth in Table 1 that the Firstthrough Ninth Preferred Embodiments of the presentsolid-polymer-electrolyte membrane could attain a high grafting rateeven by a small dose of irradiation. On the other hand, ComparativeExample Nos. 1 through 6 could not show an enlarged grafting rate. Inparticular, Comparative Example Nos. 1 and 2 employing the PTFE polymerfilm exhibited an extremely small grafting rate. Even when the dose ofradiation was increased, not only were Comparative Example Nos. 1 and 2not applied to construct a fuel cell, but also they were so brittle thatthey could not be examined for their physical properties.

Comparing the Fourth Preferred Embodiment with Comparative Example Nos.3 and 5, they had a substantially identical grafting rate. However, theFourth Preferred Embodiment exhibited a lower specific resistance thanthose of Comparative Example Nos. 3 and 5. Thus, the Fourth PreferredEmbodiment was apparently suitable for constructing a fuel cell.

Moreover, the results of the evaluation recited in Table 1 show that theFirst through Ninth Preferred Embodiments of the presentsolid-polymer-electrolyte membrane had such a strength that they couldsatisfactorily be applied to construct a fuel cell. Comparative ExampleNos. 1 through 6 had an inadequate strength so that they were inadequatefor constructing a fuel cell.

The aforementioned advantages of the First through Ninth PreferredEmbodiments are believed to be effected by the following reasons.

When the copolymer, made from the fluorocarbon-based vinyl monomer andthe hydrocarbon-based vinyl monomer is irradiated, the fluorinated mainchains of the copolymer are predominantly subjected to scission at thecarbon-carbon bonds to produce radicals. The polymerizable alkenylbenzene is graft-polymerized into these radicalized segments. Upon theirradiation, the unfluorinated main chains of the copolymer arepredominantly subjected to cleavage at the carbon-hydrogen bonds toproduce radicals. In addition to the graft polymerization reaction withthe polymerizable alkenyl benzene, these radicals were likely tore-combine, and form cross-linked constructions intermolecularly. Thus,in the radiation-graft polymerization of the presentsolid-polymer-electrolyte membrane, the radicals resulting from thefluorinated main chains, and the radicals resulting from theunfluorinated main chains and capable of cross-linking are producedsimultaneously. As a result, the present solid-polymer-electrolytemembrane can presumably maintain its strength. Contrary to the presentsolid-polymer-electrolyte membrane, in the graft polymerization reactionof conventional solid-polymer-electrolyte membrane, there exist noradicals capable of intermolecularly forming the cross-linkedconstructions, because the main chains of conventionalsolid-polymer-electrolyte membrane are completely fluorinated. Hence,the main chains of conventional solid-polymer-electrolyte membrane aresimply disconnected to low-molecular weight fractions in operating fuelcells, and sulfonic groups attached to the main chains are easily lost.Thus, conventional solid-polymer-electrolyte membrane cannot keep itsstrength and performance at all.

Evaluation of Output Voltage Performance

Thus prepared the Sixth and Seventh Preferred Embodiments of the presentsolid-polymer-electrolyte membrane, Comparative Example No. 3, and acommercially available Nafion membrane were utilized to construct a fuelcell as illustrated in FIG. 1. The Nafion membrane had a thickness of100 μm, and exhibited an ion-exchange capacity of 0.91milli-equivalent/g.

As illustrated in FIG. 1, the fuel cell included asolid-polymer-electrolyte membrane 1, gas diffusion electrodes 2, 3holding the membrane 1 therebetween, electricity collectors 4, 4 holdingthe gas diffusion electrodes 2, 3 and the membrane 1 therebetween. Thegas diffusion electrodes 2, 3 were formed of carbon black and atetrafluoroethylene resin. The electricity collectors 4, 4 were formedof carbon. In the interfaces between the gas diffusion electrodes 2, 3and the membrane 1, there was coated platinum in an amount of 0.4mg/cm².

The component parts (e.g., the membrane 1, the gas diffusion electrodes2, 3, and electricity collectors 4,4) of a fuel cell were wrapped with afluorocarbon-resin sheet, a filter paper, a stainless plate, and afilter paper in this order. The thus wrapped component parts were placedon a hot pressing machine whole temperature had been adjusted to 100° C.in advance. The component parts were subjected to a pressure of 20kgf/cm² at 100° C. for 5 minutes. Then, the component parts were furthersubjected to a pressure of 20 kgf/cm² at 132.5° C. for 5 minutes. Thecomponent parts were furthermore subjected to a pressure of 20 kgf/cm²at 165° C. for 5 minutes. Thereafter, the pressure was increased to 80kgf/cm², and was held thereat for 90 seconds. A plurality of fuel cellsfor the evaluation of output voltage performance were thus prepared.

The fuel cells were evaluated by using a testing apparatus asillustrated in FIG. 2, thereby determining their polarization curves.

The testing apparatus illustrated in FIG. 2 was operated as hereinafterdescribed. A hydrogen gas and an oxygen gas were generated by means ofwater electrolysis by using a water-electrolysis gas generator 11. Thegenerated oxygen gas, and the generated hydrogen gas were transferredthrough pots 12, 13, respectively. The oxygen and hydrogen gasesabsorbed water at the pots 12, 13. The oxygen and hydrogen gases withwater absorbed were supplied to a fuel cell 10, respectively. Theexcessive hydrogen gas could be collected in the pot 14, and theexcessive oxygen and the water produced could be discharged from anoutlet port 15. The pots 12, 13 could be heated so that it was possibleto control the water vapor content in the oxygen and hydrogen gases tobe supplied to the fuel cell 10. A replenishing pot 16 could replenishthe pot 13 with water. In order to inhibit the hydrogen gas fromreacting with oxygen included in air, the pot 13 was connected with anitrogen-gas bomb by way of a tube 17 so that nitrogen gas could beintroduced into the pot 13 from the nitrogen-gas bomb.

A predetermined load was applied to the fuel cell 10 in order to observethe output voltage variation, and the polarization curve of the fuelcell 10 was determined. FIG. 3 shows the relationships between the load(or electric current density) and the output voltage. In FIG. 3, thesolid diamonds (♦) specify the polarization curve obtained in Test No. 1in which the fuel cell was constructed by using the Sixth PreferredEmbodiment of the present solid-polymer-electrolyte membrane; the blanksquares (□) specify the polarization curve obtained in Test No. 2 inwhich the fuel cell was constructed by using the Seventh PreferredEmbodiment of the present solid-polymer-electrolyte membrane; the blankcircles (◯) specify the polarization curve obtained in Comparative TestNo. 1 in which the fuel cell was constructed by using ComparativeExample No. 3 of conventional solid-polymer-electrolyte membrane; andthe solid squares (▪) specify the polarization curve obtained inComparative Test No. 2 in which the fuel cell was constructed by usingthe commercially available Nafion membrane. Note that, in Test Nos. 1and 2 as well as in Comparative Test Nos. 1 and 2, the used gasdiffusion electrodes had a surface area of 10 cm² ; platinum andruthenium were loaded as catalysts in the interfaces in an amount of0.77 mg/cm² and 0.23 mg/cm², respectively; the fuel cells were operatedat a temperature of 70° C.; and the hydrogen gas, and the oxygen gaswere supplied to the fuel cells at 1 atm, respectively.

It is apparent from the results illustrated in FIG. 3 that the fuelcells employing the preferred embodiments of the presentsolid-polymer-electrolyte membrane did not exhibit a sharplydeteriorating output voltage even when the electric current density wasincreased, and that they exhibited a high output voltage at any electriccurrent density.

On the other hand, when the electric current density was zero, the fuelcells exhibited a relatively high output voltage (i.e., an openingoutput voltage) in Comparative Test Nos. 1 and 2. However, when theelectric current density was increased, the fuel cells exhibited a loweroutput voltage than those of the fuel cells prepared in Test Nos. 1 and2.

Durability Test

Except that the electric current density was fixed at 0.7 A/cm², thefuel cells prepared in Test No. 1 and Comparative Test No. 1 wereoperated in the same manner as Test No. 1 above, and were examined forthe durability. FIG. 4 illustrates the results of this durability test.

It is appreciated from FIG. 4 that the fuel cell employing the SixthPreferred Embodiment of the present solid-polymer-electrolyte membraneexhibited a stable output voltage for a long period time. After thedurability test, the fuel cell was disassembled to examine theappearance of the membrane. As a result, no change was found in theappearance of the membrane, and the ion-exchange capacity did not showany change.

On the other hand, the fuel cell employing Comparative Example No. 3 ofconventional solid-polymer-electrolyte membrane apparently exhibited adeteriorating output voltage as the time elapsed. In fact, afteroperating the fuel cell for 20 hours, the fuel cell did not generateelectricity at all.

Having now fully described the present invention, it will be apparent toone of ordinary skill in the art that many changes and modifications canbe made thereto without departing from the spirit or scope of thepresent invention as set forth herein including the appended claims.

What is claimed is:
 1. A process for producing asolid-polymer-electrolyte membrane for a polymer-electrolyte fuel cell,comprising:irradiating a film-shaped copolymer made from afluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer,the main chain of which has formula (1): ##STR5## wherein R¹ is afluorine atom or a fluoro-C₁₋₃ -alkyl group, R² is a hydrogen atom orC₁₋₃ -alkyl group, m is an integer of 1 or more, and n is an integer of1 or more, and thereafter contacting a polymerizable alkenyl benzene offormula (3): ##STR6## wherein each of R³, R⁴, and R⁵ is a hydrogen atomor a C₁₋₃ -alkyl group, and t is 0 or an integer of 1 or more with theirradiated polymer, thereby forming a graft side chain resulting fromthe polymerizable alkenyl benzene; and introducing a sulfonic acid groupinto the resulting graft side chain.
 2. The process for producing asolid-polymer-electrolyte membrane for a fuel cell according to claim 1,wherein, in said irradiation step, said film-shaped copolymer isirradiated at a dose of from 1 to 100 kGy.
 3. The process for producinga solid-polymer-electrolyte membrane for a fuel cell according to claim1, wherein said radiation-graft polymerization step is carried out at atemperature of a boiling point or less of said polymerizable alkenylbenzene.
 4. The process for producing a solid-polymer-electrolytemembrane for a fuel cell according to claim 1, wherein saidsulfonic-group introduction step is carried out by using afluorosulfonic acid or a chlorosulfonic acid.
 5. A process for producinga solid-polymer-electrolyte membrane for a polymer-electrolyte fuelcell, the process comprising:irradiating a film-shaped copolymer madefrom a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinylmonomer, the main chain of which has formula (1): ##STR7## wherein R¹ isa fluorine atom or a fluoro-C₁₋₃ -alkyl group, R² is a hydrogen atom orC₁₋₃ -alkyl group, m is an integer of 1 or more, and n is an integer of1 or more, and thereafter contacting a polymerizable alkenyl benzenehaving a sulfonic acid group of formula (4): ##STR8## wherein each ofR³, R⁴, and R⁵ is a hydrogen atom or a C₁₋₃ -alkyl group, and t is 0 oran integer of 1 or more with the irradiated polymer, thereby forming agraft side chain resulting from the polymerizable alkenyl benzene havinga sulfonic acid group.
 6. The process for producing asolid-polymer-electrolyte membrane for a fuel cell according to claim 5,wherein, in said irradiation step, said film-shaped copolymer isirradiated at a dose of from 1 to 100 kGy.
 7. The process for producinga solid-polymer-electrolyte membrane for a fuel cell according to claim5, wherein the formation of said graft side chain is carried out at atemperature of a boiling point or less of said polymerizable alkenylbenzene having a sulfonic group.